Positron emission tomography (PET) is an imaging method to obtain quantitative molecular and biochemical information of physiological processes in the human body. The most common PET radiotracer in use today is [18F]-fluorodeoxyglucose ([18F]-FDG), a radiolabelled glucose molecule. PET Imaging with [18F]-FDG allows to visualize glucose metabolism and has a broad range of clinical indications. Among positron emitters, [18F] is the most widely used today in the clinical environment.
[18F] fluoride is produced by irradiation of water (containing H218O) with protons resulting in the reaction 18O(p,n)18F. Only a minor fraction of the [18O] is converted. For production efficiency and radiochemical purity, it is desirable to use water that is as highly enriched as possible. The physics of the production of [18F] fluoride by proton bombardment of water (amount of heat, proton energy range) typically requires, at least 1 mL of water. The volumes coming out of most cyclotron targets are in practice several mL.
The [18F] isotope is then separated from water and processed for production of a radiopharmaceutical agent. Conventional fluoride recovery is based on ion exchange resins. The recovery is carried out in two steps (extraction and elution): first the anions (not only fluoride) are separated from the enriched [18O] water and trapped on a resin and then, said anions, including [18F] fluoride, are eluted into a mixture containing water, organic solvents, a base, also called activating agent or phase transfer agent or phase transfer catalyst, such as for example the complex potassium carbonate-Kryptofix 222 (K2CO3-K222) or a tetrabutylammonium salt. The [18F] fluoride radiochemical recovery yield is very effective, usually exceeding 99%. The most usual labelling method, known as nucleophilic substitution, requires anhydrous or very low water content solutions and whatever the method used, an evaporation step always follows the recovery of the [18F]fluoride. It usually consists in multiple azeotropic evaporations of acetonitrile or low boiling temperature organic solvent, that require several minutes.